Problem: In the right triangle shown, $AC = 4$ and $BC = 5$. What is $AB$ ? $A$ $C$ $B$ $4$ $5$ $?$
We know $a^2 + b^2 = c^2$ We want to find $c$ ; let $a = 5$ and $b = 4$ So $c^2 = 5^2 + 4^2 = 41$ Then, $c = \sqrt{41}$.